I got home from work Thursday a little after 7pm and the power was out. The interior was at 21 degrees when I ran the slides out and turned the furnace on. By 11 when we went to bed it was 66 degrees in the camper with just the furnace. I hooked my truck up to the camper battery using jumper cables and let it run for about an hour before going to bed to charge the battery. The overnight low was 17 degrees and I was impressed that the furnace kept up with no electric heaters. The low the night before was -3 degrees so I'm glad it wasn't Wednesday night.